The invention was developed in connection with an underground test facility for recovering bitumen from subterranean oil sands. Whilst the invention will be described in connection therewith, it is not limited in application to that environment.
The oil sands involved are located in Northern Alberta and are buried under an overburden of varying thickness.
In those areas where the oil sand is close to the ground surface, the overburden is stripped off and the oil sand is mined with draglines. The oil sand is then conveyed to an extraction plant in which the bitumen is separated from the solids, using a process referred to in the industry as the hot water process.
However, most of the oil sand is buried too deeply to be processed in the manner just described. In this latter circumstance, the procedure used has involved drilling wells from the surface and introducing heat to the formation, through the wells, to reduce the viscosity of the bitumen and render it mobile. The heated bitumen is then produced through the same or other wells and brought to the surface.
There are many problems associated with the recovery of bitumen using thermal processes operated through wells extending from ground surface. These problems have been so severe that this technique is limited to commercial application in only the deepest, thickest and richest sections of the Alberta oil sands.
The present assignee, a research agency of the Alberta government, initiated a novel approach toward recovering bitumen from the buried oil sands. As a first step, two laterally spaced apart, vertical, large diameter shafts were drilled and cased from ground surface into the limestone formation underlying the oil sand. Horizontal tunnels or drifts were then extended from the shafts through the limestone using conventional blast-and-remove mining technique. The drifts connected the two vertical shafts, so that ventilation was secured. Steam injection and bitumen production wells were then drilled upwardly from the drifts and completed in the oil sand.
The recovery process applied involved emplacing a fluid production well which extended horizontally through the oil sand for 60 meters, generally parallel and close to the oil sand/limestone interface. A generally co-extensive steam injection well was run essentially parallel to and about 3 to 7 meters above the production well.
An extensive programme of geotechnical instrumentation was also undertaken. The programme was designed to measure the temperature, pore pressure displacement and (by inference) the effective stress fields within the steaming zone and in the formation adjacent to the tunnels.
Heating of a formation causes volume changes in both the mineral particles and the bulk matrix of the solids and in the pore fluids. These volume changes lead to deformations, changes in the formation stresses and to increases in the pore fluid pressures. Instruments known as piezometers (or pore pressure transducers) are utilized to measure pore pressure.
In conventional geotechnical engineering practice, piezometers are installed as follows. The instrument is first lowered on its cable to the desired depth in the borehole. A sand pack is then positioned around the piezometer so as to provide an efficient hydraulic connection between the latter and the formation. The sand pack is isolated, or sealed, above and beneath by means of bentonite pellets, grout or the like. This above-described method is quite practical when only a single piezometer needs to be installed and when the boreholes are shallow.
However, there are problems that arise when the boreholes are greater in depth than about fifty feet. Additionally, it becomes difficult when one seeks to install multiple piezometers to determine pore pressures at differing elevations in a single borehole. Exemplary difficulties involve space limitations or the inaccuracy of placement of the instruments at the predetermined depth because of the cable having become twisted or ensnarled. Furthermore, installing a multiplicity of piezometers at various depths is time-consuming and laborious because it is necessary to permit the seals above and beneath each sand pack plus piezometer to set prior to the insertion of each successive instrument. Disadvantageously, this prior art system lacks flexibility if, for example, it is found that after grouting an instrument is dysfunctional and it is not possible to remove it from the borehole for replacement.
There exists, therefore, the need for a method for installing a multiplicity of instruments in a single borehole and for a device therefor, characterized by the following advantages:
rapidity and ease of installation; PA1 accuracy in depth placement; and PA1 mechanical simplicity and ruggedness. PA1 Providing an inner support pipe which may be interconnected with a string of grouting pipe, so as to form a string having a continuous fluid passageway for introduction of grout to the bottom of the borehole, said string including the support pipe as an integral part thereof; PA1 Providing at least one tray member which is pivotally associated with the support pipe. The tray is provided with spring means which normally seek to expand to pivot the tray so that it will move radially outwardly from the support pipe into a second deployed position wherein its contents pressingly engage the borehole wall. The tray is further provided with means for restraining it against the support pipe, with the spring means compressed, in a first stowed position, so that the unit is sufficiently compact for entry into the borehole. The restraining means are adapted to be released from the surface; PA1 The tray supports a piezometer, which piezometer is embedded in a pre-formed sand pack carried by the tray. The sand pack provides a hydraulic connection for the piezometer with the borehole wall. A large contact area with the borehole wall is provided by the sand pack, to ensure adequate communication with the formation. Preferably, the sand pack comprises sand and an epoxy compound in admixture, which mixture has been allowed to set; PA1 Preferably, a second tray is associated with each support pipe in tandem with the first tray. The second tray is movable outwardly to contact the borehole wall, in the same manner as the first tray. The second tray functions as a guide for the cables of piezometers positioned deeper in the borehole. Thus, the cables are less likely to become entangled. Preferably, the second tray is provided with serrations along its outer edges which, when the trays are deployed, grip the borehole wall so as to prevent the device from becoming dislodged during the grouting operation. PA1 inserting a plurality of piezometers, each embedded in a sand bed and carried by a tray, into the borehole, said tray being in a stowed position on a string of grouting pipe, said tray having means, which may be actuated from the surface, for moving the tray outwardly and radially from the pipe to cause the sand pack to contact the borehole; PA1 optionally, testing all the piezometers prior to deploying the trays; PA1 actuating the trays so as to bring the sand pack into hydraulic connection with the formation; and PA1 then grouting the borehole in a single operation so as to hydraulically isolate the piezometers at differing elevations from one another.